1. Field of the Invention
This invention relates to a microfilm projection apparatus for projecting an image in a microfilm on an image receiving surface by the use of the zoom lens selected from a plurality of zoom lenses.
2. Description of the Prior Art
U.S. Pat. No. 4,172,657 discloses a reader printer capable of being fitted up simultaneously with a plurality of mono-focal lenses of severally different magnifying powers. Though this reader printer is allowed to alter the magnifying power in projection stepwise by the selection of particular mono-focal lenses to be used, it is not allowed to alter continuously the magnifying power in the projection of a microfilm. Therefore, a zoom lens is used, for the purpose of allowing the magnifying power in the projection of a microfilm to be continuously altered. The range in which the magnifying power attained by one zoom lens is limited. To allow a wide range for the magnifying power, the practice of preparing a plurality of zoom lenses having severally different ranges of magnifying power and selectively using a particular zoom lens whose range of magnifying power embraces a desired magnification.
Incidentally, in the field of microfilms, fixed standards are applied to the ratios of reduction and sizes in which microfilms are to be manufactured. When the plurality of zoom lenses are prepared as described above, however, only a few magnifying powers are actually used because the number of magnifying powers frequently used with one zoom lens is fixed in a certain sense. Even in setting such magnifying powers of high frequently in a given zoom lens, the operator is required for each change in magnification to perform a zooming operation on the zoom lens so as to set the prescribed magnifying power.